Directional coupler



Jan. 22, 1957 J. K. RECORDS 2,779,001

DIRECTIONAL COUPLER Filed May 28, 1952 T0 T0 MEASURING MEASURING APPARATUS F APPARATUS a T 6/ 33 EH f m g f 5 lrfiventor:

John K. -Reczords,

b MEW His Attorney.

United States Patent DIRECTIONAL COUPLER John K. Records, North Syracuse, N. Y., assignor to General Electric Company, a corporation of New York Application May 28, 1952, Serial No. 290,405

8 Claims. (Cl. 333-10) My present invention relates to the art including ultrahigh-frequency wave-transmission apparatus, and more particularly to such apparatus that is responsive to the direction of flow of wave energy. More specifically, this invention is concerned with an improved coupler device having substantially increased directivity.

Heretofore, directional-coupler apparatus have been described in which one or more auxiliary conductors are coupled to a main wave-conductor section by means rendering the apparatus responsive to the direction of energy flow in the main conductor. In such prior known apparatus, it is customary to provide terminating impedances for the auxiliary conductors, which impedances have been carefully selected to match the characteristic or surge impedance of the respective auxiliary conductors so as to prevent or minimize reflection of energy incident on the end of the auxiliary conductors at which the terminating impedances are located.

For monitoring power transmitted over the main conductor section between a source of ultra-high-frequency energy and a utilization device therefor, conventional directional coupler devices, as for example, the three-hole binomial coupler described in the book Principles and Applications of Waveguide Transmission by G. C. Southworth, published by D. Van Nostrand Company, Inc., New York, 1950, have sufiiciently good directivities, as measured by the ratio of the incident power applied to the main conductor section to the backward power. As is well known, the directivity D, in decibels, is given by the equation D log gi where P1 is the incident power and Pb is the backward power.

However, for high-accuracy voltage-standing-wave-ratio measurements, the inherent directivites of prior known couplers are found to be insuflicient for the reflected wave coupler, and it is, accordingly, a principal object of this invention to provide improved directional coupler apparatus responsive to the direction of energy flow along an ultra-high-frequency conductor in which substantially increased directivity is obtained.

In the past, attempts at improving the directivity of directional couplers have consisted mainly in the provision of means to improve the matching of the terminating impedances in the auxiliary conductors to the characteristic impedances of the respective conductors. Such improvements have resulted merely in approaching more closely the inadequate inherent directivity of the coupler.

Incarrying out my invention in one form, I improve the directivity of a reflected-wave coupler by effectively cancelling the reverse or backward wave that is set up in the main conductor as a result of inherent errors in mismatch of the load thereto and/or errors in the constructional design of the coupler. Such constructional design errors may originate in the spacing and dimensioning of the coupling means employed between the main and auxiliary conductors.

"ice

. connection with the accompanying drawing, in which Fig.

l is a plan view, with parts broken away, of a directional coupler constructed in accordance with my invention; and Fig. 2 is a side elevational view of the coupler, with a part broken away.

In Fig. l, a directional coupler, according to my invention, is shown comprising a length or section of ultrahigh-frequency wave-energy conductor 11, which may be of the well-known hollow waveguide type, as shown, or any other suitable conducting means, as for example, a section of coaxial conductor or parallel wire transmission line. Section 11, here shown as of rectangular cross section, can be of any other desired cross section, such as circular or elliptic. The section 11 is adapted to be connected at one end 13 to a source (not shown) of ultra-high-frequency energy and at the other end 15 to a utilization device (not shown) therefor. The respective ends 13 and 15 are, therefore, provided with suitable choke-flange couplings, as required, for joining the section 11 to the source and utilization device with minimum leakage of energy. Section 11 is commonly referred to as the main conductor or guide of the directional coupler, the principal direction of flow of energy therethrough being from end 13 toward end 15, as indicated in the drawing by the solid arrow 17.

A pair of auxiliary waveguide sections 19 and 21, having propagation characteristics similar to those of the main guide 11, extend axially along portions of opposed side walls 23 and 25, respectively, of the main guide 11, the auxiliary guide 19 sharing the wall 23 in common with the main guide 11, and theauxiliary guide 21 sharing the opposite wall 25 in common with guide 11.

Electrical wave coupling between the main waveguide 11 and the auxiliary guides 19 and 21 is effected by means of a series of apertures 27, 29 and 31 in the wall 23 and a similar series of apertures 33, 35 and 37 in the wall 25. The first-named series of apertures operates to couple a part of the energy from the main guide 11 to the auxiliary guide 19, and the second series of apertures serves to couple a part of the energy in the main guide to the auxiliary guide 21. The spacing between the apertures is substantially one quarter wavelength at the frequency of operation of the coupler, and the relative dimensions of the apertures of each series are determined in accordance with the coefficients of the binomial expansion, substantially as described in the abovementioned book of Southworth.

The auxiliary guide 21 is termed the forward-wave coupler since, with energy flowing in the main guide 11 from left to right, as indicated by the arrow 17, the energy coupled into the auxiliary guide 21 through the apertures 33, 35 and 37 adds in the direction parallel to the arrow 17, as indicated by the solid arrow 39 and cancels in the opposite direction. The energy in the auxiliary guide 21, the so-called forward-coupled energy, can be measured by any suitable measuring apparatus connected to an output coupling 41 in the form of a coaxial probe, as illustrated in Fig. 2, of any well-known type, which projects into the guide 21 through the wall 43 thereof.

The auxiliary guide 19 is usually termed the reflectedwave coupler, inasmuch as wave energy traversing the main guide 11 from right to left, as indicated by the dotdash arrow 45, is partly coupled into the auxiliary guide 19, adding in the reverse direction, indicated by the dotdash arrow 47, andcancelling in the opposite or forward 1.3 direction. The reverse direction of flow in the main guide may be set up, for example, by reflections from the utilization device, by transmission discontinuities due to errors in the spacing and/ or dimensioning of the coupling apertures and by other systematic errors in construction of the transmission system. The energy in the auxiliary guide, the so-called reverse-coupled energy, can be measured by any suitable measuring apparatus connected to an output coupling 49 of a type similar to the probe coupling 41 in auxiliary guide 21. The complete apparatus comprising the main and associated auxiliary waveguides is commonly known as a bi-directional coupler, inasmuch as the apparatus is responsive to energy flow in both the forward and reverse directions.

Terminating impedances 51 and 53 are provided in the reverseand forward-wave couplers 19 and 21, respectively, and can be in the form of respective pairs of tapered strips 52, 54 and 56, 58 of resistive or semiconductive materials composed of carbon, graphite, silicon carbide or other non-metallic materials combined in a suitable bonding agent or carrier. Any desired angle of taper may be employed, as shown in Fig. 2, to provide a good match for the operating band of frequencies.

As shown, each pair of strips is preferably mounted parallel to each other on respective backing plates, one such plate 55 being adapted to close the right-hand end of the reverse-wave coupler 19, the other plate 57 forming a closure member for the left-hand end of the forward-wave coupler 21. Mounting of the strips 52, 54 and 56, 58 on the respective plates 55 and 57 can be by any suitable manner, as by riveting the strips to tabs 60, which, in turn, may be soldered or otherwise mounted on theinner faces of the plates. The plates together with the strips can be removably connected to the associated coupler, as by machine screws 59. In this manner, damage to the strips by heat required in brazing the coupler assembly can be avoided since the strip sub-assemblies can be connected after the brazing is completed.

Screws 61, 63 are threaded in the upper wall 65 of the reflected-energy coupler 19 and project into the space between the strips 51 for altering the admittance of the guide for purposes presently to be described.

As noted above, the forward-wave coupler 21 is commonly used to monitor the energy propagated in the forward direction for power measurements or the like, and, when terminated in strips 56, 58 having an impedance value matching the characteristic impedance of the guide 21, this part of the coupler possesses a directivity sufficiently good for monitoring.

For very accurate Voltage-standing-wave-ratio measurements, however, the inherent directivity of the directiona1 coupler is not suificiently high even when the impedanceof the terminating strips 52, 54 in the reflected-energy coupler 19 is perfectly matched to the characteristic impedance of the guide section 19. The limit of the inherent directivity appears to be determined by the precision with which directional coupler theory predicts the performance of a given coupler, the accuracy with which the coupling apertures can be formed and spaced, and the constancy of the operating frequency. It has been observed that with the main guide section 11 terminated in approximately its characteristic impedance so that the voltage standing wave ratio is less than 1.01, part of the energy coupled from the main guide section 11 into the reflected-wave coupler 19 is measurable at the coupling 49, thusindicating a directivity of less than 46, db.

To obtain substantially increased directivity, the reverse-coupled energy is substantially cancelled by setting up a reflection from the terminating impedance 51 by means of the adjustable screws 61, 63, which operate to mismatch the impedance of the termination with respect to the characteristic impedance of the waveguide section 19. In this manner, forwardly-directed energy in the main guide 11, coupled into the auxiliary guide 19, in

stead, of being completely absorbed inthe matched term;

mination 51, is partly reflected therefrom, the amplitude and phase of the reflected component depending on the degree of insertion of the screws 61 and 63 between the strips 52 and 54. Proper adjustment of the screws thus results in the production of a wave having an amplitude equal to the amplitude of the reverse-coupled waves and 180 out of phase therewith. The combination of these waves results in a null indication on the measuring apparatus connected at the output coupling 49, the directivity thus becoming greater than or equal to 46 db.

' In a practical embodiment of my invention, designed for operation at a mid-band frequency value of 2880 megacycles per second, a directivity of 46 db. or more was obtained as compared with a directivity of about 30 db. obtainable without the cancellation means according to this invention.

It will be understood that the novel features of my invention can be attained with directional couplers other than the binomial coupler herein disclosed, as for example the two-hole, slotted and/ or loop couplers well known to those skilled in the art. The specific type of coupler described is to be regarded as illustrative only and not as limiting the scope of my invention.

While I have shown and described specific embodiments of my invention, I do not desire my invention to be limited to the particular form shown and described and I intend by the appended claims to cover all modifications within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A directional coupler, comprising a first waveguide section adapted to propagate electromagnetic wave energy, a second waveguide section having an output portion, means including a common wall for said waveguide sections having a plurality of spaced apertures therein for interconnecting said sections, whereby part of said energy is directionally coupled into said second section, said coupler normally having imperfect directivity in eifecting energy transfer to said output portion, and terminating means in said second section presenting an impedance mismatch with respect to said second section, whereby to reflect part of the coupled energy in predetermined amount and phase, to said output portion,

- said terminating means being adjustable to establsh said predetermined amount and phase in such a way that the effect of said reflected part of said coupled energy is to cancel the directivity-reducing energy at said output portion.

2. A directional coupler, comprising a first waveguide section adapted to propagate electromagnetic wave energy, second and third waveguide sections each having output portions therein, means including common wall portions between said second and third waveguide sections, respectively, and said first waveguide section, said wall portions having a plurality of spaced apertures therein for interconnecting said sections, part of said energy being directionally coupled thereby into said second and third sections, one of said second and third sections being adapted for transferring energization to the output portion thereof by forward-traveling waves in said first section, the other being adapted for transferring energization to the output portion thereof by reverse-traveling waves in said first section, said coupler normally having imperfect directivity in effecting energy transfer to said respective output portion, resistive means terminating said one section for substantially reflectionless absorption of wave energy therein, and impedance means terminating said other section and having an impedance characteristic mismatched relative to that of said other section, whereby to reflect part of the energy incident thereon in predetermined amount and phase to cancel the directivity-reducing energy at the output portion thereof.

3. A directional coupler, comprising a first waveguide section adapted to propagate electromagnetic wave energy, a second waveguide section havingan, output.

portion, means including a common wall for said waveguide sections having a plurality of spaced apertures therein for interconnecting said sections, whereby part of said energy is directionally coupled into said second section, said coupler normally having imperfect directivity in effecting energy transfer to said output portion, and terminating means in said second section adapted to reflect part of the coupled energy to said output portion in predetermined amount and phase to cancel the directivity-reducing energy at said output portion, said terminating means comprising a pair of spaced parallel resistive members having an impedance value substantially equal to the impedance of said second section, and adjustable reactance means disposed between said resistive members for altering the impedance match between said second section and said resistive members.

4. The directional coupler as defined in claim 3, further comprising detachable closure means at the end of said second section adjacent said terminating means, tabs projecting from said closure means, and means to affix said resistive members to said tabs.

5. A directional coupler, comprising a first waveguide section adapted to propagate electromagnetic Wave energy, a second waveguide section having an output portion, means interconnecting said sections, whereby part of said energy is directionally coupled into said second section, said coupler normally having imperfect directivity in effecting energy transfer to said output portion, and reactive terminating means in said second section having an impedance characterisic mismatched relative to that of said second section, whereby to reflect part of the coupled energy in predetermined amount and phase to cancel the directivity-reducing energy at said output portion.

6. High-frequency apparatus, comprising a first waveguide adapted to be coupled to a source at one end and to a utilization device at the other end, a second Waveguide adjacent said first waveguide having an output portion, means directionally coupling said waveguides in energy-exchanging relation, the energy in said second waveguide supplied to said output portion being principally derived from a flow of energy in said first waveguide in a direction opposite the normal direction of flow in said first waveguide, said apparatus normally having imperfect directivity in efiecting energy transfer to said output portion, and means terminating said second waveguide and having an impedance value differing from that of said second waveguide by a predetermined amount, whereby to provide a reflected component of the energy in said second waveguide at said output portion having equal magnitude and opposite phase relative to that portion of the forward-directed wave energy in said first waveguide which is coupled into said second waveguide, said reflected component being adapted to cancel the portion of forward-directed wave energy at said output portion, thereby to increase the directivity of 5 said apparatus.

7. Directional-coupler apparatus, comprising a main waveguide adapted to be coupled to a source at one end and to a utilization device at the other end, an auxiliary waveguide having a wall portion in common with said main waveguide and an output portion, said common wall portion having apertures directionally coupling said waveguides in energy-exchanging relation, whereby the output portion of said auxiliary waveguide is principally energized in response to flow of energy in said main waveguide in a direction opposite the normal direction of flow, said apparatus normally having imperfect directivity in efiecting energy transfer to said output portion, spaced terminating impedance elements in said auxiliary waveguide, and adjustable reactance means comprising a pair of adjustable screws mounted for projection between said impedance elements and adapted to provide a reflected component of energy of magnitude and phase respectively equal and opposite to the reversedirected wave energy in said auxiliary waveguide at said output portion, whereby to cancel the reverse-directed wave energy.

8. High-frequency apparatus, comprising a first waveguide adapted to be coupled to a source at one end and to a utilization device at the other end, a second Waveguide having a wall portion in common with said first waveguide and an output portion, means in said common wall portion for directionally coupling said waveguides in energy-exchanging relation, the output portion of said second waveguide being principally energized by a flow of energy in said first waveguide in a direction opposite the normal direction of flow therein, said apparatus normally having imperfect directivity in efli'ecting energy transfer to said output portion, and adjustable means terminating said second waveguide in an impedance mismatch to provide a reflected component of energy at the output portion of said second Waveguide having equal magnitude and opposite phase relative to that portion of the forward-directed waves in said first waveguide which are coupled to said output portion of said second waveguide, said reflected component being adapted to cancel the reverse-directed waves at said output portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,544,842 Lawson Mar. 13, 1951 2,567,210 Hupcey Sept. 11, 1951 2,580,678 Hansen Jan. 1, 1952 2,618,744 Braden Nov. 18, 1952 2,636,116 Taylor Apr. 21, 1953 2,697,208 Houghton Dec. 14, 1954 OTHER REFERENCES Publication I, Grantham, A Reflectionless Wave- Guide Termination, Review of Scientific Instruments, vol. 22, No. 11, November 1951, pp. 828-834. 

